Synthesis of Crown Ethers for Use in Studies of Fluorescence
An Honors Thesis (Honors 499)
by
Melissa K. Slater
-
Thesis Advisor: Dr. Lynn Sousa
Ball State University Department of Chemistry
Muncie, IN
Date: May 8, 1997
Date of Graduation: May 10, 1997
L
Abstract: The following thesis is an overview of the undergraduate research I performed during
my last three semesters. I worked with Dr. Lynn Sousa in the area of Organic
Chemistry, trying to synthesize, identify, and isolate a crown ether molecule that would
be used in studies of fluorescence.
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SYNTHESIS OF CROWN ETHERS
FOR USE IN STUDIES OF
FLUORESCENCE
Melissa Slater
Senior Honors Thesis
Faculty Advisor: Dr. Lynn Sousa
Ball State Department of Chemistry
1
Introduction
I began working with Dr. Sousa during the summer of 1996 as part of the chemistry
department's Undergraduate Summer Research Program. It is a ten week program in which
professors allow students to aid them with their independant research projects or develop one of
their own. It gives students the opportunity to gain valuable experience in a number of areas in
chemistry.
Dr. Sousa's main area of interest is Organic Chemistry. He had been working on a
particular project for several years, and wanted a summer research student to continue with one
particular idea he had been working on.
I was available for research the first five-week session of the program, and paired up with
Dr. Sousa because he was also available then. We were both able to commit to the project on a
part-time basis. Then in the fall of 1996 and spring of 1997 I continued doing research with Dr.
Sousa part time. This thesis is an overview of what I have done.
Background
Organic Chemistry is the chemistry of carbon-containing compounds and their properties.
It is also called the "chemistry of life" because all living things contain carbon and carbon-based
chemistry. A feature unique to the element carbon is the fact that it can bond strongly to itself,
creating straight and branched carbon chains and intricate ring structures up to great sizes:
-
2
H
HI H
H
H
H
I
I
I
I
H-C-C-C-C-C-H
I
I
I
I
I
H H H H H
pentane
I
H, . . . C, . . . H
C
'C
I
,C
~ C'
H
I
'H
H
anthracene
C14 H 1O
CSH12
Many organic compounds have functional groups, or reactive areas on the molecule that
give it specific properties. One common functional group in organic chemistry is called an ether,
which consists of an oxygen atom that has a carbon on both sides.
r-\
HI H
HI HI
I
H-C-C -O-C -C-H
I
I
I
I
H H
H H
diethyl ether
(0 0)
CH 3 0CH 2 CH 3
ethyl methyl ether
o
\..0
0
oj
'--'
18-crown-o
1 .4,7,10,13, 16-hexaoxacyclo-octadecane
In 1967, Peterson first reported the existence of a new type of ether. Peterson's ether was
actually a ring of ethers, the 18-crown-6 shown above. It was the first time a cyclic polyether of
this type had been synthesized. Since then, a great amount of research has been dedicated to
synthesizing and studying molecules of this type. A specific characteristic of crown ethers is their
ability to complex or hold a positively-charged ion( cation) in their cavities. The attraction and
complexation between the two is referred to as host-guest chemistry. The host crown can be one
of several variations of the original 18-crown-6, and the guest is typically an alkali metal cation.
Attaching a chromophore, a large ring structure that absorbs specific frequencies of light, causes
the total molecule to fluoresce when energy is focused on it. Several studies have been devoted to
the idea that heavier complexed cations such as cesium seem to quench fluorescence, while
-
potassium does not stop the fluorescence.
In 1977, Dr. Sousa and his students first reported the observation of alkali-metal enhanced
3
fluorescence of a 21-crown-6 attached to a naphalene chromophore. In 1981, they began to design
and synthesize compounds that would display this type of cation-enhanced fluorescence.
Specifically, Dr. Sousa wanted to synthesize a chromophore-bearing crown ether that might have
fluorescence sensitive to potassium ions. There are relatively few compounds with attached
chromophores that display potassium-enhanced fluorescence specifically. The chromophore(flat
ring structure) would form the flat bottom of the molecule, with the crown open and extending
over the top as a "strap" or "jaw" that would hold the round cation in between the two. The
resulting molecule would resemble a "basket", with the chromophore bottom and the crown
"handle" .
The purpose of creating a molecule of this type would be for biological studies and testing.
If the molecule were to be found extremely specific in complexing potassium as opposed to other
metal ions, the fluorescence given off would be a good measure of the presence or amount of
potassium in a biological sample. The fluorescence of this molecule would provide an alternate
method of looking at potassium concentrations.
While Dr. Sousa successfully synthesized and studied this single-strap molecule, in recent
years he has given thought to the idea that where one strap binds the cation in place, two straps
might be more effective in promoting potassium-enhanced fluorescence. The goal of my research
was to synthesize, identify, and isolate a larger crown ether (to form a double strap), and attach it
to a chromophore for use in fluorescence studies.
Focus and Primary Goal:
1\/\/\
H0>C0
H
0
0
0
O~/OH
0
0
O~H
LJ'--I"-I
cis and trans
1,4,7,10,14, 17 ,20,23-octaoxa-12,25-dihydroxycyc1ohexacosane
1\1\/\
O~O
0
0
O~O
"'--0
0
0
O~
LJ'--I"-I
1,4,7,10,14,17 ,20,23-octaoxa-12,25-dioxocyc1ohexacosane
-
4
The focus of my research was these two crown ether compounds. The left molecule is the
dihydroxy form of the crown, with alcohol end groups. The alcohol end groups are the part of the
molecule that would allow the attachment to the chromophore to take place. When attached, the
middle of the crown would bend upwards, forming the set of straps over the molecule:
The molecule to the right above is the diketone form of the crown, having the end two carbons
double-bonded to oxygens. The dike tone form would have to be reduced to the dihydroxy form
before it could be attached. Since the reduction of the diketone could be done fairly easily, we
pursued both types of the crown.
I. The Research Plan
Attempting to synthesize either crown ether involved several steps:
1. Envision the compound we wanted to make
2. Search the STN scientific computer databases to see if the compound had been made
previously
3. Conduct a literature search for similar molecules and syntheses of similar molecules to
use as guidelines for our reactions
4. Attempt to synthesize a crown using the literature precedence
5. Determine if the product of the reaction contains something that behaves like the crown
--
5
6. Attempt to separate the reaction product into molecules of different sizes and further
isolate the desired crown
7. Determine and confirm which separate fraction contains the crown
8. Attach the crown to the chromophore for future study by Dr. Sousa
We began our research by conducting a computer search through STN, a worldwide
scientific database that indexes molecules and scientific publications. After searching for the
existence of both molecules, we found that neither had been previously made. If we had found
one, our research would have consisted of synthesizing it according to the literature, then attaching
it to the chromophore. However, since neither had been made our task was to be considerably
larger.
--
The next step required an extensive search through the scientific literature. I needed to find
reactions that resulted in molecules similar to the crown we were looking for. When I found
something we might be able to apply, we would alter the procedure to favor the large type of
product we desired. I spent several weeks looking for procedures, eventually finding a reaction
that might be used in synthesizing each of the crown ethers.
II. Attempted Reaction #1
We decided to attempt to make the diketone form of the crown first. We modified a
procedure that originally formed a smaller diketone crown, using triethylene glycol and 1,3dichloroacetone in a 2:2 ratio according to the following reaction:
-
6
=CO
+
OH
f\/\/\
r\r\r\
0
0
OH
CI
1,3-dichloroacetone
O~O
0
0
"-0
0
0
DMF
O~O
O~
LJ'---!L.J
triethylene glycol
1,4,7, I 0, 14, 17 ,20,23-octaoxa-12,25-dioxocyclohexacosane
Experimental
A 250-ml round-bottom flask was fitted with a dry stopper, reflux condenser, and a
gas inlet. Argon was run through the inlet and out the top of the reflux condenser.
1,3-Dichloroacetone (3.80g,30mmol) was weighed under the hood and quickly
added to the flask, then II.4g(35mmol) of CS2C03 was added. Triethylene glycol
(4.51,30mmol) and 100 ml dry DMF(driedover sieves) were quickly added,
creating a dark brown solution. The solution was stirred several minutes then
heated by heating mantle to a gentle boil.
The solution was boiled for
approximately 48.5 hours, then removed from heat and stirring. The solution was
....-
a dark brown color with fine white sediment in it. After cooling, the sediment was
separated by vaccuum filtration, washed with 5xlOml portions of dichloromethane,
then isolated and dried, resulting in 9.78g of solid. The liquid was transferred to a
250-ml round bottom flask and rotary evaporated to remove dichloromethane, and
the DMF and remaining dichloromethane were removed by vaccuum distillation at
-28°C and 1.33 mmHg. The result was a thick, dark brown oil only partially
soluble in deuterochloroform.
Reference used: J. Org. Chern. 1994,59,2186-2196. by Gibson et al.
The difficulty in synthesizing crowns is the fact that many sizes form in a given reaction.
The reaction would naturally tend to form the smaller, easier to form I: 1 product. While we may
purposely use conditions favoring the formation of the larger crowns, it is highly probable that
smaller ones form from different ratios.
-
7
~~~
+
0==<:",,0
°
°
°=>=0
0'--..../0'--..../0'--..../0
etc.
2:2
To take a look at what different molecules the oil might contain, I first employed a
technique called Thin-Layer ChromatographYa (see appendix)' The resulting TLC's did not show the
type of separation we had expected. In fact, it was difficult to tell if we had separation at all. The
next step was to take a proton NMRa to look for the presence of hydrogens on a molecule of our
type. The spectrum did not indicate the correct ratio of hydrogens that would be found on our
crown. After closely examining the mechanism of the reaction, we realized that it was possible for
the starting materials to undergo the Favorski Rearrangement, which would result in a completely
different product than desired. Since Dr. Sousa felt that the Favorski Rearrangemet was a definite
threat to successfully synthesizing the diketone crown, we decided to move on to the dihydroxy
crown.
III. Attempted Reaction #2
We then attempted to synthesize the dihydroxy form of the crown. With a more solid base
in the literature, we followed a reaction that had resulted in a good amount of fairly large dihydroxy
crown. Again, we altered the procedure to promote formation of a larger molecule, using 1,3dichloro-2-propanol and triethylene glycol.
CI
HO--C
CI
1,3-dichloro-2-propanol
.-
/\/\/\
+
OH
0
0
triethylene glycol
OH
KH
THF
•
H0>C 0
H
0
I\r\r\
°
0
0
0
O~/OH
O~H
'--!'-1L.J
cis and trans 1,4,7, I 0, 14, 17,20,23-octaoxa-12,25-dihydroxycyclohexacosane
8
Experimental
Potassium hydride, KH (8.03g of 35% KH in oil suspension) was weighed and
added to a three-necked, 500-ml round bottom flask. The slurry was washed with
4x30ml of dry THF. THF (100 ml) was added to the flask, and 4.81g(.032mol) of
triethylene glycol in 50 ml dry THF was placed in a closed addition funnel on the
middle neck of the flask. The triethylene glycol was added dropwise, with stirring,
over several minutes, to produce a tan slurry. The flask was then heated to gentle
reflux. 1,3-Dichloro-2-propanol (4.0g,.031mol) in 50 ml dry THF was added
dropwise through the funnel. The slurry was kept at reflux(with stirring) under
Argon for approximately 15 hours, resulting in a dark tan solution. Stirring was
continued after removal of the heat source. After cooling, 5 ml methanol was added
to destroy any remaining KH, then the reaction mixture was filtered through celite
and washed with THF. The THF was rotary evaporated off to leave 9.62g of dark
brown, thick oil. This oil was lighter and thinner than the previous reaction
mixture, and also completely soluble in deuterochloroform.
Reference used: Liebigs Ann. Chern. 1985,210-213 by Ulrich et al.
The result of this reaction was a lighter, thinner oil than previously obtained in the diketone
synthesis. Silica Gel TLCa now indicated that we had created a mixture of different molecules.
The molecules tended to travel at two rough speeds. To separate the reaction mixture into
molecules of different size, I employed a technique called column chromatographYa. I then studied
the fractions collected from the alumina column separation for signs of the the crown. Again,
TLC's of several fractions over time showed evidence that we had two main sizes of compounds.
The faster-moving molecules were evident in the beginning fractions, and a slower-moving spot
mainly appeared in the later fractions.
Proton and Carbon NMRa were used to study the makeup of the molecules in the main
groups of fractions. NMR MS-22-1a is an example proton NMR spectrum of an early fraction,
fraction 8, believed to contain the bulk of the desired compound. The peak that would be
atttributed to hydrogens of crown compounds is where it was expected. The corresponding carbon
NMR, spectrum also showed evidence of a crown. While there is clear evidence that triethylene
9
glycol, one of the starting materials, still remains, there is also a new peak that cannot be attributed
to anything but the possible formation of the new crown.
NMR's MS-22-3a and MS-22-3b are again proton and carbon NMR's, respectively, this
time for a later fraction. You can see that there is more triethylene glycol (see NMR MS-17 -5) in
this fraction, and also evidence of the new peak. MS-22-3b also shows what is called an Attached
Proton Test(APTh. Looking at the target crown ether, we would hope to see all peaks pointing up
except one. The signal from the carbon with the -OH and H group attached would point down.
On the upper spectrum of MS-22-3b, that is exactly what we see. While this is not cleat proof of
our crown, it is promising.
Close to the end of the semester, I attempted another separation of the original reaction
mixture, this time using a silica gel column. However, the results of the separation were not very
clear. After examining TLC and NMR data, it appears that most of the component of interest did
not leave the column as expected.
IV.
Conclusion
We are unable to say if we have obtained the dihydroxy crown. The separation by the
alumina column seemed successful, and the NMR spectra were promising. However, more
research needs to be done before it can be confirmed either way.
Doing research has been a rewarding experience for me. I have learned a lot of valuable
techniques in the lab, as well as organization and how to manage my time efficiently. I am glad I
was able to be a part of the undergraduate research program. I feel I have learned many valuable
things I can apply to my future.
-
10
Appendix
Attached Proton Test (APT) - a test that can be run on the NMR that looks at the number of
hydrogens attached to the carbons in your molecule. If the peak is pointing up, the carbon
that peak represents has either 0 or 2 attached hydrogens. If the peak is pointing down, the
carbon has 1 or 3 hydrogens.
Column Chromatography - method of separation involving an open glass tube with a valve at the
bottom. The column is packed with alumina or silica gel, then a layer of sample is
introduced at the top. Solvent is added through the top and allowed to run out the bottom,
taking different components of the original mixture through with it. The speed of the
different molecules depends on their polarity and size, so by taking several fractions off the
bottom of the column it is possible to separate your mixture.
Nuclear Magnetic Resonance (NMR) - an instrument used to qualitatively analyze organic samples.
The spectrum and integration obtained can suggest the number and ratio of different
hydrogen and carbon types in your sample.
Thin Layer Chromatography - a method of separation involving a thin layer of alumina or silica gel
on a glass or plastic plate. A small drop of the sample is spotted onto the plate, and the
plate is then set upright in a shallow amount of solvent in an airtight container. Because the
components of the mixture have different polarities and solubilities, they are carried up the
plate and spread differently as the solvent front rises. It is used to separate and visualize
the different components of the mixtue.
-
11
References
Bloomfield, Molly M. Chemistty and the Living Organism, 2nd Ed; John Wiley & Sons: New
York,1980.
Braun, Robert D. Introduction to Instrumental Analysis; McGraw-Hill: New York, 1987.
Cram, Donald J; Trueblood, Kenneth N. Concept. Structure. and Binding in Complexation. In
Host Guest Complex Chernistty: Macrocycles; Vogtle, F; Weber, E., Ed.; SpringerVerlag: Berlin, 1985.
Ebbing, Darrel D. General Chemistty; Houghton-Mifflin Company: Boston, 1984.
Skoog, Douglas A. Principles of Instrumental Analysis; Harcourt Brace: Orlando, 1992.
Zumdahl, Steven S. Chernistty, 2nd Ed; D.C. Heath and Company: Lexington, MA, 1989.
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